You're not going to find a lot of cover selections for the large format Boox, they just isn't the market for it. I would suggest a good stand, that you can use with pretty much any cover, or coverless.
I have a couple of these that I bought from Amazon. Very sturdy, and multiple angles viewing. As shown in the pic, it folds down flat. The base also adjusts, so I can use it with my M90 and all of my other ereaders.
